It is often the case that when an intended measurement is being made, the sensing device also responds to unintended physical variables or properties. For instance, a strain gauge instrument may be applied to determine the weight bearing on a mechanical member, but if the instrument is not designed properly, it may also be measuring the mechanical member's temperature. In some situations, such as with incremental measurements or time aspects being measured, the intended measurement can be separated from unintended measurement, because the time scales differ. For the above example, temperature changes may take place on a scale of hours, whereas relevant weight-bearing load changes may occur on a scale of seconds. In this case, the relevant weight-bearing load measurements could be changes in load as opposed to absolute values and/or frequency and/or other time patterns of these changes.
If the effects cannot be separated by time scale or frequency spectrum, additional sensors can be added that measure only the unintended variable. For the above example, the temperature response of the strain gauge instrument can be determined, and a temperature sensor can be added to determine the true weight bearing load, regardless of temperature changes.
For some years, there have appeared on the market a large number of products that monitor vital signs to track disease states. More recently, there has been a shift in emphasis to monitor the vital signs of individuals for whom there is no suspected illness, but for early indications of health problems. Examples of such vital signs are heart rate and motion, blood pressure, and respiration rate. Part of the need is to provide measurements for a period of hours that may span a variety of activities, such as relaxation, physical exercise, and stressful mental activities. Such measurements can be recorded automatically for later analysis, and in some cases, even be accompanied by alarms when there are readings that do not fall within selected limits.
Also, during the past several years, there have been many products introduced to allow monitoring of infants and others for reasons of safety. Parents and others are able to monitor subjects who are located in different rooms of a dwelling or in completely different locations.